I propose: 1. To use a double sucrose gap technique to record from and voltage clamp small strips or trabeculae (diameter equals 250 microns, length equals 3-4mm) of tissue which are dissected from the sinus venosus region of the heart of the American bullfrog, Rana catesbeiana. The preparation and certain necessary refinements of this method have been developed within the last two years. Preliminary experiments strongly indicate that the proposed work will provide significant advances in the understanding of the mechanism of generation of the pacemaker depolarization in primary pacemaker tissue. 2. To make use of this voltage clamp data in order to study the actions of the autonomic transmitters on individual time- and voltage-dependent ionic currents in sinus tissue. 3. To collaborate with Dr. John Blinks in injecting aequorin into frog sinus cells in an attempt to test the possibility that a calcium influx is involved in the generation of the pacemaker potential. 4. To utilize field stimulation and microelectrode recordings from an isolated rabbit sinus node preparation in order to carefully re-assess the electrophysiological changes produced by excitation of the cholinergic or sympathetic nerve endings.